Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), non-volatile, floating gate NOR/NAND flash memory, and dynamic random access memory (DRAM).
Flash memories may use floating gate technology or trapping technology in order to store data in the form of charges. Floating gate cells include source and drain regions that are laterally spaced apart to form an intermediate channel region. The source and drain regions are formed in a common horizontal plane of a silicon substrate. The floating gate, typically made of doped polysilicon, is disposed over the channel region and is electrically isolated from the other cell elements by oxide. The non-volatile memory function for the floating gate technology is created by the absence or presence of charge stored on the isolated floating gate.
The trapping technology functions as a non-volatile memory and can be implemented in a silicon-oxide-nitride-oxide-silicon (SONOS) architecture or nano-crystal devices. The nitride trap or nano-crystal layer can capture and store electrons or holes that have tunneled through the tunnel insulator in order to act as a non-volatile memory. These types of devices are typically referred to as discrete trap or embedded trap devices.
Conventional DRAM cells are comprised of a switching transistor and an integrated storage capacitor tied to the storage node of the transistor. Charge storage is enhanced by providing appropriate storage capacity in the form of a stacked capacitor or a trench capacitor in parallel with the depletion capacitance of the floating storage node. DRAM cells are volatile and therefore lose data when the power is removed.
DRAMs use one or more arrays of memory cells arranged in rows and columns. Each of the rows of memory cells is activated by a corresponding row line that is selected from a row address. A pair of complementary digit lines are provided for each column of the array and a sense amplifier coupled to the digit lines for each column is enabled responsive to a respective column address. The sense amplifier senses a small voltage differential between the digit lines and amplifies such voltage differential.
Due to finite charge leakage across the depletion layer, the capacitor has to be recharged frequently to ensure data integrity. This is referred to in the art as refreshing and can be accomplished by periodically coupling the memory cells in the row to one of the digit lines after enabling the sense amplifiers. The sense amplifiers then restore the voltage level on the memory cell capacitor to a voltage level corresponding to the stored data bit. The permissible time between refresh cycles without losing data depends on various factors, such as rate of charge dissipation in the memory capacitor, but is typically in the range of milliseconds.
Computers, cell phones, and many other hand-held electronic devices employ several types of the above memories for working memory and data store. These memories require custom technologies that are typically not compatible to each other due to different cell design, fabrication techniques, and material characteristics. Consequently, the different memories are produced on different silicon substrates to minimize cost and maximize product yield.
Both DRAM and floating gate flash consume relatively high power compared to other memory technologies. DRAM requires frequent refreshing to maintain the data integrity while flash memory requires on-chip high voltage/current for programming and erase operations.
Another problem with these technologies is scalability. The DRAM has capacitor scalability problems while the flash has voltage and coupling noise scalability problems. Additionally, with progressive scaling of feature size, fundamental device leakage issues such as short-channel effects and gate dielectric leakage will need to be contained in order to take advantage of scaling.
To solve some of these problems, single transistor SONOS/nano-crystal devices have been used. However, these types of devices can exhibit limited retention and small values for the memory window, thus limiting their application potential and scalability for non-volatile memory. This is due to the fact that nitride layers provide relatively shallow trap depth and nano-crystals provide low trap density due to coulomb blocade and quantum confinement effects. The threshold window of memory devices using nano-crystals is also adversely affected by the separation of the nano-crystals if the relative distances between the nano-crystals are random.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a discrete trap, multi-functional memory device that incorporates nano-crystals having uniform distribution and size with a high density.